It has been common practice to package dairy products and fruit juices in gable top cartons which are constructed of a laminate having a paperboard substrate that is coated on both sides with a thermoplastic material, such as polyethylene. In order to prevent the migration of oxygen into the interior of the package, the laminate may include a barrier layer of aluminum foil or other suitable material between the substrate and the polyethylene liquid contact layer. The carton is formed from a carton blank into the proper shape and then the bottom seams are bonded together by heat sealing. During the filling operation, the juice or other liquid product is introduced into the carton through the open top. The carton is not completely filled, so that when the top is closed, there is a space above the liquid which is referred to as the headspace. The top of the carton is closed by folding opposite side walls at the center toward each other, while moving the other two side walls toward each other to form the top which resembles a roof gable. The top edges of all four sides overlap each other at the top of the gable and are heat sealed together under pressure. The headspace contains air or other gas that is trapped inside the carton when the gable top is closed and sealed.
Juice products, such orange juice, can be stored at room temperature in these cartons for substantial periods of time if microorganisms are excluded from the interior of the carton, or are killed by conventional techniques, such as heating, hydrogen peroxide treatment, or ultraviolet light treatment. Form, fill and seal processes and apparatus, such as the Tetra Brik system that is described, for example, in U.S. Pat. Nos. 4,384,438 and 4,464,156, have been used successfully for packaging juice and liquid food products in paper-coated cartons in which the liquid completely fills the package. This system, however, is not suitable for packaging liquid food products in gable top cartons.
Various processes have been proposed for filling gable top cartons with juice products under conditions which prolong the shelf life of the product, but these processes have the disadvantage of not producing a product having favorable taste, vitamin C retention, color or filling characteristics. There are two processes that have recently been proposed for packaging orange juice in gable top cartons that have a long shelf life at room temperature. The first of these processes is known as the post packaging thermal process (PPTP). The second process is known as the "hot fill" process.
The PPTP process, which is described in Canadian Patent No. 1,290,972, dated Oct. 22, 1991, involves filling a gable top carton with juice at room temperature. The carton is then sealed after it is filled. The filled and sealed carton, after being removed from the filling and sealing machine, is heated to a sufficiently high temperature and for a sufficiently long time to kill all of the bacteria in the carton. This heating step takes between 10 and 12 minutes. The filled cartons are then cooled to room temperature and may be stored without refrigeration for as long as three months.
In the Hot Fill process, unpasteurized juice is heated to about 95.degree. C. The hot juice is then dispensed into the open top of a gable top carton. The filled cartons are sealed while the juice is maintained at 95.degree. C. The cartons are then inverted to ensure that the headspace of the cartons is sterilized by the hot juice. The cartons are then cooled to room temperature over a period of about one-half an hour.
These processes have various disadvantages in the packaging of high acid liquid food products, particularly orange juice. The PPTP process requires filling and sealing the cartons at room temperature. The cartons are then heated to at least 75.degree. C. for 10 to 12 minutes to sterilize the contents. When the sealed cartons are heated, both the product and the carton expand, which places a substantial stress on the packaging material, and may cause leakage. Further, the PPTP process takes about 35 minutes to complete the heating, holding and cooling steps.
When the cartons that have been filled according to the Hot Fill process or the PPTP process have cooled, there is a tendency for the product in the cartons to contract. This contraction creates a vacuum in the carton, which often causes the shape of the carton to be distorted. Another problem with the Hot Fill process is that orange juice has a substantially greater tendency to form foam when filling at elevated temperatures, so that it is difficult to precisely control the fill level in the carton and the foam interferes with the sealing of the top of the carton. Further, when the carton and the contents are heated to a temperature as high as 95.degree. C., the strength of the seal at the top of the carton is substantially less than at lower temperatures. Another disadvantage of the use of these processes is that volatile flavor components are lost at a greater rate when the contents of the cartons are heated to a temperature of 95.degree. C.
When packaging orange juice, it is advantageous to use a packaging laminate that includes a barrier layer which will prevent the loss of essential oils and flavor components in the juice. Typically, aluminum foil is used for this purpose. Recently, ethylene vinyl alcohol copolymers have been substituted for aluminum foil in these packaging laminates. In order to resist distortion of the package upon cooling, additional layers of polyethylene have been included in the laminate. This increases the cost of the laminate, which reduces the financial benefit of using paper-based packaging. Another problem that occurs upon cooling the packages that are produced by these processes is that a vacuum is created in the headspace which makes the carton difficult to open and which may be sufficient to cause the seal in the top fin to leak. Ambient air that leaks into the carton will cause premature spoilage of the orange juice.